1. Field of the Invention
The present invention relates generally to electrooptical devices for directing the propagation of light energy and, more particularly, concerns light guiding apparatus for the amplitude modulation of light flowing in optical fiber light propagating systems.
2. Description of the Prior Art
In recent research directed to the development of guided wave optical modulators suitable for direct insertion in fiber optic communication systems, emphasis has increasingly been centered upon the generation of active components propagating all or most of the multiple mode energy characteristic of optical fiber propagation, thus avoiding the difficult problems associated with coupling such fibers to single mode active devices. Several concepts found in these programs demonstrate merit, for example, as optical guides. In general, these optical propagation devices employ a fixed voltage placed across an electrooptic crystal to produce an increase in the index of refraction of the crystal in a fashion such as to enhance propagation of light therethrough. If the crystal is instead subjected to a varying voltage, the voltage-induced wave guide serves as an amplitude modulator, since the strength of the light passed through the crystal will then depend upon the strength of effectiveness of the induced wave guide. With a large applied guiding voltage, the light output is strong; no guiding voltage or a reversed electric field results in a correspondingly small optical output because the light is not efficiently guided from the input to the output port.
One such high speed modulator is described by the present inventor and his co-workers D. H. McMahon and R. L. Grave in the technical paper: "Electro-Optic Channel Waveguide Modulator for Multimode Fibers", Applied Physics Letters, Vol. 28, No. 6, Mar. 15, 1976, pages 321 through 323. This prior art device consists of a thin transparent plate of Z-cut electrooptically active LiTaO.sub.3 with opposed conductive strip electrodes evaporated on the opposed broad faces of the plate. Opposed edges of the plate are polished so that optical fibers may be butt coupled to form input and output ports for the modulators. The strength of the electric field readily controls the strength of effectiveness of the wave guiding channel lying between the two electrodes. The guiding region is desirably formed by the electric field to have substantially the same cross-section dimension of the active core of commercially available multimode optical fibers, permitting efficient coupling therebetween of the modes propagating within the optical fiber guides.